1. Field of the Invention
The present invention relates to a method and an apparatus for the purification and treatment of liquids which are used for cooling and/or lubricating and contain impurities of metal and other materials, particularly for separating scale from the process water of cooling cycles in the metallurgical industry.
2. Description of the Related Art
The metallurgical industry utilizes large quantities of cooling water and lubricants, particularly in continuous casting plants and hot-rolling plants. In addition to scale, the industry produces metal chips and waste from flame cutting and flame chipping. In large blooming trains in hot-rolling mills, the scale is produced in the form of flat cake-like scale or fish scale, while on roller conveyors and cooling beds the scale is produced in the form of small plates or platelets down to grain sizes of a few thousandths of a millimeter. The cooling water supplied to the rolling process is advantageously simultaneously used as a conveying means for the rolling scale and other solids being produced, and for washing away the lubricants, such as oil or grease, which are used. The process water is collected and supplied to a treatment cycle. Particularly problematic for the treatment and purification of the process water has been found to be the fact that the process water with the scale particles becomes mixed with oils and greases which makes it necessary to provide substantial devices for carrying out the treatment and recovery process.
In a conventional plant, the purification and treatment requires a sequence of individual process steps in separate plants, beginning with a coarse purification in a sedimentation basin. In this deep basin, the so-called scale pit, in which the cooling water returning, for example, from a rolling mill is collected, the larger and, thus, heavier scale particles which contain relatively little oil, are separated as a result of sedimentation. The cooling water which has been prepurified in this manner is subsequently pumped into a pretreatment basin in which a further separation of solids is achieved by the use of gravity. For carrying out a subsequent fine purification, the water is pumped through gravel or sand filters and reaches a collecting basin through recooling structures and is then returned to the rolling mill by means of process pumps.
Particularly disadvantageous in this conventional plant are the large space requirement of the plant, the high investment costs and the large amount of energy required for the intermediate pumps.
Another disadvantage is the fact that the work of the pumps causes the leakage oil quantity produced in the rolling mill to adhere to the fine scale which is separated partially in the pretreatment basin and partially in the filters, so that the scale becomes useless and cannot be returned to another metallurgical process. Only the coarse scale which is already separated in the deep basin is still essentially free of oil components and, thus, is suitable for a metallurgical process.
Instead of gravel or sand filters, centrifugal separators are also known in the art for fine purification. In these separators, the liquid and the matter suspended in the liquid enters tangentially and is placed into a circulating movement. Those solids which are heavier than the liquid are forced by the centrifugal force against the wall of the separating chamber and the solids slowly slide downwardly into a collecting chamber. A problem is the fact that the pumping work requires a large amount of energy. Moreover, the effectiveness of this method is limited by the particle sizes; this is because the centrifugal force must increase with decreasing size of the particles and the pumping power and the energy requirement are also increased. In addition, this method of cleaning does not reach the cleaning efficiency which can be achieved by gravel or sand filters.
It is known from DE 41 15 819 A1 to carry out a fine purification by means of a magnetic separator installed in a subsequently arranged pump unit. In accordance with this method, in a zone of the basin of the pump unit which has as little flow as possible, midsized and fine scale particles are separated from the process water by means of the magnetic separator and are stripped above the water level from the magnetic separator. The scale particles obtained in this manner are subsequently returned to the metallurgical process. No details are provided with respect to the type of magnetic separator.
A magnetic filtering plant for waste water purification is disclosed in the ASEA Journal, 1978, 23rd year, Vol. 1, pages 15-21. The proposed purification plant utilizes magnetic disks which are arranged parallel to each other and rotate in a basin which contains the water to be purified. The suspended particles adhere to the disks solely as a result of magnetic force; the particles are stripped from the surfaces of the disks by means of endless profiled rubber belts and are transported out of the filter. However, the endless rubber belts for stripping and removing the sludge are subjected to substantial wear and must be replaced regularly.
In addition to the techniques for the purification of liquids described above, also known in the art is the technique of using parallel plates with inclined lamellae. In accordance with this technique, suspended substances are separated from the liquid by means of gravity sedimentation, wherein the solid particles are deposited on the plates during an upward flow produced in the liquid and slide downwardly into a sludge funnel in a counter-current flow to the upwardly moving clear water. The thin sludge collected in this manner is removed by means of pumps and is further thickened. To ensure that the solid particles slide downwardly, the plates must have an inclination which frequently is 70.degree. relative to the horizontal. However, when the angle of inclination is increased, the efficiency of the separation as a result of sedimentation on the individual plates is reduced. As is the case when separating by means of centrifugal force, this technology does not reach the high degrees of purity as they are achieved when purifying by means of gravel and sand filters. The limit of the separation capability is determined by the density and the size of the solid particles.